Distillation

Distillation separates the parts of a liquid by heating it until part of it turns to vapour, then cooling that vapour back into liquid in another vessel. Described in Roman Egypt around 300 CE, it has given pure water, perfumes and essential oils, alcohol for medicine and, since the nineteenth century, the fuels refined from crude oil.

What it is

Distillation separates substances by evaporating and condensing them. A liquid is heated, usually to boiling, until part of it turns to vapour, the vapour is led away and cooled, and it becomes liquid again in another vessel. Whatever does not evaporate stays in the pot: salt, mud, dyes, most germs.

Apothecaries used it for medicines and scented waters, perfumers to take the fragrance out of flowers, physicians to make the strong alcohol they used as a solvent and on wounds. Laboratories and hospitals use distilled water because it carries almost no dissolved minerals, and lead-acid batteries are topped up with it for the same reason.

The plate shows a pot still of the early sixteenth century, as drawn in the first printed books on distilling: a copper pot on a small furnace, a domed head, and a pipe that winds through a tub of cold water before dripping into a flask.

How it works

Every liquid gives off vapour, more of it as it gets hotter, and it boils when the pressure of that vapour equals the pressure of the air. At sea level water boils at 100 °C, ethanol, the alcohol in wine, at about 78 °C, and methanol at about 65 °C. Salts barely evaporate at all, so boiling seawater and condensing the steam gives fresh water.

The fire heats the pot and the vapour rises into the head. It runs down a pipe into the worm, a coil standing in cold water, which takes its heat away. The vapour condenses inside the copper and trickles out into the receiving flask.

In a Florentine flask the oil floats on the water, which drains away through the side tube.

Essential oils are taken out by steam distillation. Steam passed through flowers, leaves or peel carries away their fragrant oil. Oil and water do not mix, so each adds its own vapour pressure and the pair boils below 100 °C, even when the oil alone would boil above 250 °C. The oil comes over without scorching and settles as a separate layer in the receiver.

When two liquids in the pot both evaporate and mix, as water and alcohol do, the separation is only partial. The vapour is richer in the more volatile one, so distillers passed the product through the still again. A fractionating column repeats this many times in one pass: rising vapour meets liquid running back down, called reflux, and at each level part of it condenses and boils again. Refineries use such columns to split crude oil into gas, petrol, kerosene, diesel and heavy residue by boiling range.

How to build

This section covers one thing only: drinkable water from salty or muddy water. It does not describe making alcohol. Distilling spirits at home is illegal without a licence in many countries, and a badly run still can concentrate methanol, which blinds and kills in small amounts and cannot be detected by taste or smell.

Solar still. Put a few centimetres of the water in a shallow dark basin in full sun. Cover it with clean glass or with clear, heat-tolerant food-grade plastic, tilted so that drops run down its underside to a gutter along the low edge, which leads to a covered glass jar. Seal the edges so that vapour cannot escape. In tropical sun a well-built still gives about two to three litres per square metre a day; expect less elsewhere, and less again with makeshift materials.

With the lid upside down, the water that condenses on it runs to the handle and drips into the bowl.

Pot and lid. Half fill a large metal pot and set it over a fire. Stand a small clean metal bowl on a stone in the middle, its rim well above the water. Put a metal lid on upside down, handle pointing at the bowl, and keep it cool with snow, cold water or a wet cloth; never cool a glass lid this way, as it can shatter. Keep a gentle boil: hard bubbling splashes raw water into the bowl.

Watch for leaks. Dry a few drops of the collected water on a clean dark plate. A white crust means raw water is splashing or leaking into the collector. A clean plate proves nothing more: it shows neither that the still separates well nor that the water is safe, since clear water can carry harmful chemicals and only a laboratory test can tell.

Know the limits. Distillation leaves behind salt, metals such as lead, nitrate and germs. It does not reliably remove substances that boil near or below the boiling point of water, such as some solvents and fuels, and some other chemicals are carried over with the steam: if the source may be chemically polluted, find another. Store the water in clean, covered glass, stainless steel or food-grade plastic containers, because germs return easily. Avoid glazed pottery, whose glaze may release lead.

Safety. Steam scalds worse than boiling water: lift lids away from your face. Never seal a vessel that is being heated or tie its lid down; the vapour needs a way out, or the pot can burst. Ordinary glass cracks on a flame or under sudden cold. Watch the fire, keep it away from dry grass and keep children at a distance. Never distil fuel, alcohol or unknown liquids: their vapours catch fire, and any methanol in a fermented liquid comes over into the distillate.

History and evolution

At Tepe Gawra in northern Iraq, pots of the fifth and fourth millennia BCE have a channel around the rim that may have collected condensed liquid. Replicas tested in experiments published in 2020 produced scented water, but their original use is still debated.

The tribikos of the Alexandrian alchemists sent the vapour through three spouts into three flasks.

The first clear descriptions of stills come from Roman Egypt. Around 300 CE the alchemist Zosimos of Panopolis wrote about stills with one or several pipes and credited some of them to Maria the Jewess. Alchemists and physicians of the Islamic world took up the apparatus, and its Arabic name, al-anbiq, became “alembic”. In the ninth century the philosopher al-Kindi wrote a book on perfume and distillation with over a hundred recipes.

In Bologna in the 1270s, physicians described aqua vitae, “water of life”, alcohol distilled from wine and used as a remedy. In Strasbourg the surgeon Hieronymus Brunschwig published the first printed manual on distilling medicinal waters in 1500, then a larger book in 1512.

In 1830 the Irishman Aeneas Coffey patented a two-column still that ran continuously instead of in batches. Around 1854 Samuel Kier of Pittsburgh ran one of the first oil refineries, distilling crude oil into lamp kerosene. In 2011 distillation still accounted for about a third of installed desalination capacity worldwide, often in plants that use the waste heat of power stations.